In Japan the NTSC television broadcasting system is adopted at present. Recently, recently a wide aspect screen high precision HDTV broadcasting has been started. In the HDTV broadcasting, the aspect ratio of the screen differs from that of the NTSC system. In addition, a television receiver with a 16:9 aspect ratio screen which is wider than the 4:3 aspect ratio screen of the current NTSC television receiver has been adopted. The Television receiver down-converts the HDTV broadcasting signal to the current NTSC system broadcasting signal to enable viewers to enjoy the HDTV broadcasting program on the NTSC system television receivers. Even in this case. To ensure viewing of full image information of the HDTV signal, the 16:9 wide aspect picture tube may be used for receiving the NTSC signal.
When images with the 4:3 aspect ratio are displayed on the 16:9 aspect ratio screen television receiver, the following display modes are available: A first display mode to display a 4:3 aspect ratio image on a central area of a wide aspect picture tube by compressing the horizontal time base of the NTSC signal; A second display mode to display a aspect ratio image on the full area of the 16:9 wide aspect ratio picture tube without compressing the horizontal time base, but expanding the vertical amplitude.
The second display mode is suited for displaying a movie software. That is, as shown in FIG. 1(a), when a movie is displayed on a 4:3 aspect ratio screen, it is displayed at a movie size aspect ratio. Thus, black blank areas are left on the top and bottom of the screen. However, if the movie is displayed on the 16:9 wide aspect picture tube by expanding the vertical amplitude, the black blank areas are removed from the screen while the movie picture is fully expanded over the screen, as shown in FIG. 1(b).
FIG. 2 shows a conventional circuit for expanding the vertical amplitude. In FIG. 2, reference 10 denotes a vertical sawtooth waveform generator, reference 20 denotes a vertical deflection circuit, reference Q1 denotes a switching transistor for switching the display modes. A mode control from a control circuit (not shown) is input to the base of the switching transistor Q1. For example, a low level signal is input to the transistor Q1 in the display mode to expand the vertical amplitude. A high level signal is input in the normal amplitude display mode.
The vertical sawtooth generating wave circuit 10 includes a ramp (i.e., sawtooth wave) generator 11 and an amplifier circuit 12. A vertical sync signal Vsync is supplied to the ramp generator 11. A capacitor C1 for charging and discharging is connected between a voltage source V1 and the ramp generator 11.
The ramp generator 11 synchronizes with the vertical sync signal Vsync and generates a sawtooth signal with an amplitude that is defined in accordance with the charging/discharging operations of the capacitor Further, the collector of the transistor G1 is connected with the ramp generator 11 via a resistor R1. Thus, in response to ON/OFF of this transistor G1, the charging/discharging level of the capacitor C1 varies to effect a change in the amplitude of the sawtooth signal.
The sawtooth wave output from the ramp generator 11 is amplified by the amplifier circuit 12 connected to the ramp generator 11, and then supplied to a vertical deflection circuit 20 through resistor R8 for outputting the sawtooth wave.
The vertical deflection circuit 20 is comprised of a transistor Q2 in the first stage and a complementary pair of transistors Q3 and Q4 coupled in a single ended push-pull arrangement with resistors R9 and R10 in the latter stage. The complementary transistors Q8 and Q4 coupled in the single ended push-pull arrangement in the latter state are driven by the sawtooth wave which is input to the base of the transistor Q2. Thus, a vertical deflection current is supplied to a deflection coil L1 which is connected to the complementary transistors Q3 and Q4 coupled in the single ended push-pull arrangement.
Further, a pump-up circuit 21 is provided for saving the power consumption by keeping a power supply voltage V2 to the transistors Q3 and Q4 at a high level during the vertical flyback period, while keeping it in a low level during the scanning period. Further, a block of capacitors C2, C3 and resistors R5 through R7 is connected between the deflection coil LI and the amplifier circuit 12 as a negative feedback loop of DC and AC components.
In the conventional circuit as described above, the vertical amplitude becomes normal amplitude when a high level control signal is input to the base of the transistor Q1, while it is expanded when a low level control signal is input to the base of the transistor Q1.
However, in the vertical amplitude expansion mode, pictures of a movie software can be displayed in an expanded size but the caption 50 is often superimposed on the black blank area (the blank space) at the bottom of the screen as shown in FIG. 1(b). Thus, the superimposed caption 50 is erased from the screen when the vertical amplitude is expanded.
To remove this fault, for instance, as disclosed in the Japanese Patent Application (TOKU-KAI-HEI) No. 5-64027 that has been filed previously by the applicant of this application, a method has been adopted to display a picture in a compressed form without erasing the superimposed caption by making the inclination of vertical sawtooth waveform gentle at the top and the bottom of the screen.
According to the prior art method, the superimposed caption takes a sufficient effect. However, as the caption is displayed in a reduced size, the raster density in the area of the reduced caption becomes high. Thus, a brightness of the caption excessively increases so that the screen glitters to decrease screen visibility/viewability. So, improvements in the processing of such captions are still demanded.
As described above, in the conventional television receivers, the vertical amplitude is expanded and an area superimposed with a caption is reduced. As a result, the raster density in the caption displaying area becomes high. Thus, a brightness of the superimposed caption excessively increases so that the superimposed caption has often made viewers to feel discomfortable and hard to see it.